In the layout of an imaging system, the apparent uniformity of the illumination projected onto the image plane is affected by the off-axis ability of the optical lens system to be able to gather light from the illuminated object plane scene. This is to say that, generally, the best light coupling occurs on the optical axis of the lens, while any coupling at any off-axis angle will be less good.
In the usual projection optics imaging system, this off-axis falloff does not matter unless the projection angle is wide. Such is the reason why the effect, while present, can be safely ignored in the usual photographic type application.
On the other hand, when the angle is determined to be wide: that is when the maximum off-axis imaging angle is over about 20 degrees, the off-axis nonuniformity effect does lead to deleterious overall results. Such a nonuniform condition may be further aggravated by an object plane scene projection system wherein the image receptor is particularly sensitive to variation in the average light received from the scene insofar as the desired final result is concerned. Therefore, the lens falloff becomes a problem when the intrinsic latitude of the imaging system, including the image receptor, does not have enough range to produce a satisfactory end result.